Automatic gain control (AGC) systems, in both analog and digital receivers, correct linear attenuation of demodulated signals. There are, in general, two types of conventional AGC systems: (1) non-blind and (2) blind. In the non-blind AGC system, synchronization (or training) sequences, which are embedded periodically in the transmitted data, are used to perform AGC. The use of training sequences, however, not only reduces the available bandwidth for data transmission, but also requires that both the pattern and the length of the training sequences, generally as specified by standards, be pre-programmed into the receiver. Furthermore, in digital receivers, the use of training sequences causes clock jitters which increase bit error rate (BER).
In blind AGC systems, training signals are not used. The conventional blind AGC system performs AGC by estimating the average power of the center frequency component. This method, however, is not only slow but is also computationally expensive.
FIG. 1 shows a block diagram of a conventional non-blind AGC system 100. The conventional non-blind AGC system 100 includes a gain error detection block 102 and a gain adjustment block 108. The gain error detection block 102 includes a synchronization detector 104 and a gain error evaluator 106. The gain adjustment block 108 includes a gain value controller 110.
In the AGC system 100 shown in FIG. 1, training sequences, as shown in the exemplary signal frame sequence for a non-blind AGC system shown in FIG. 2(A), are used. A training sequence contains a series of maximum and minimum possible values for demodulated baseband signals. In the gain error detection block 102, the synchronization detector 104 detects the training sequence, and the gain error evaluator 106 evaluates the gain error of the received signal. In the gain adjustment block 108, the gain value controller 110 outputs a gain value to compensate for the gain error as determined by the gain error evaluator 106. However, as shown in FIG. 2(A), the training sequence used for this system consumes a significant portion of the bandwidth. In addition, the training sequence must be pre-programmed or pre-defined in the synchronization detector 104.
FIG. 3 shows a conventional blind AGC system 120. In this system, as illustrated by the exemplary signal frame sequence for a blind AGC system in FIG. 2(B), training sequences are not used. The conventional blind AGC system 120 instead includes a bandpass filter (BPF) 122, a comparator module 124 (which includes comparator C1 126), and a low pass filter (LPF)/integrator module 127. The BPF 122 detects the power of the center frequency of the received passband signal x. The comparator C1 126 then compares the power of the band passed signal x.sub.-- bpf with an ideal average power. Then the LPF 127 (which includes negative gains G1 128 and G2 130, adders A1 134 and A2 132, and delay 136) both adjusts and smoothes the resulting output gain value from the comparator C1 126. One disadvantage of this type of system, however, is that it is very slow both in converging and in responding to abrupt changes in channel attenuation. Furthermore, computation of the BPF 122 is costly.
What is needed is a method and system for blind AGC which outperforms the conventional AGC in terms of both cost and performance.